Photoelectron emission microscope for wafer and reticle inspection

ABSTRACT

A method of inspecting and imaging substrates with an electron beam. The method can include a illuminating the substrate with a photon beam to cause photoemission of electrons. A low energy electron beam can be used to prevent or reduce positive charging of the substrate. Reflected electrons and/or emitted photoelectrons can be imaged to review or inspect the substrate.

BACKGROUND OF THE INVENTION

[0001] As the semiconductor industry shrinks the size of features onintegrated circuits, wafer fabs require higher-resolution techniques forinspecting silicon wafers and photomasks. Electron-beam tools formetrology, offline inspection, and even online inspection have reachedthe marketplace. These systems are conventional scanning electronmicroscopes in which an incident beam of high-energy electrons strikesthe wafer, causing secondary and other electrons to leave the surface.The electrons are detected, and the systems create an image of the wafersurface.

[0002] Scanning electron microscopes in the prior art have attempted tosolve the problem of charge control; i.e., preventing the accumulationof a positive charge on the surface of the wafer as secondary electronsleave the surface of the material. This is a difficult problem, and manyapproaches have been attempted.

SUMMARY OF THE INVENTION

[0003] The object of the current invention is to provide an inspectionsystem with higher resolution and greater sensitivity to differences inmaterials. We disclose a novel photoelectron emission microscope forwafer and photomask inspection, as well as for inspection for othersubstrates. In this microscope, an incident beam of photons strikes thewafer, and photoelectrons (electrons emitted via the photoelectriceffect) leave the surface. The microscope can create an image of thesurface by focusing the photoelectrons onto a detector.

[0004] The incident photons in a photoelectron emission microscopetypically have lower energy than the incident electrons in a secondaryemission electron microscope. In a photoelectron emission microscope,the incident photons may have an energy of only about 5 eV, which isonly slightly greater than the work function of the metal on the waferor photomask. As a result, the photoelectrons emitted have much lowerenergy than the secondary electrons emitted in a conventional scanningelectron microscope. The photoelectrons also have a much narrower rangeof energies, from a fraction of an eV to about 2 eV. The narrower rangeof energies in the photoelectrons gives this microscope a key advantageover a scanning electron microscope: lower chromatic aberration in theimaging optics. As a result of its lower chromatic aberration, aphotoelectron emission microscope offers higher resolution than ascanning electron microscope.

[0005] A photoelectron emission microscope can distinguish between twomaterials more clearly than a scanning electron microscope, especiallywhen the energy of the incoming photons lies between the work functionsof two materials. For example, polycrystalline aluminum has a workfunction of about 4.15 eV, silicon about 4.8 eV. If the microscopeilluminates a patterned wafer with 4.5 eV photons, the aluminum willemit photoelectrons, but the silicon won't. As a result, the image willoffer excellent contrast: the aluminum will be white and the siliconwill be black.

[0006] We disclose the use of a beam of low-energy electrons, inaddition to the beam of photons, to prevent a positive charge fromaccumulating on the wafer surface as a result of the photoelectronemission. We show that it is possible to operate the microscope in avariety of useful imaging modes based either on photoelectrons, or onlow-energy electrons reflected from the surface of the wafer, or on bothphotoelectrons and reflected electrons. Furthermore, we disclose a novelmethod of distinguishing between photoelectrons and reflected electronsbased on their angular distributions.

[0007] This novel method of distinguishing between photoelectrons andreflected electrons in a photoelectron emission microscope also hasadvantages when applied in a dual-beam secondary electron emissionmicroscope to distinguish between secondary electrons and reflectedelectrons. We therefore also disclose novel apparatus and methods forinspecting substrates with a dual-beam secondary electron emissionmicroscope.

[0008] The use of a low energy beam and a dual beam system are bothdescribed in more detail in commonly assigned co-pending U.S. patentapplications Ser. No. 09/854,332, filed May 11, 2001, and Ser. No.09/579,867, filed May 25, 2000. These patent applications are herebyincorporated by reference as though fully set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 shows a single-beam photoelectron emission microscope forimaging wafers or reticles.

[0010]FIG. 2 shows a single-beam photoelectron emission microscope forinspecting wafers or reticles.

[0011]FIG. 3 illustrates a method of imaging a wafer or reticle with asingle-beam photoelectron emission microscope.

[0012]FIG. 4 illustrates a method of inspecting a wafer or reticle witha single-beam photoelectron emission microscope.

[0013]FIG. 5 shows a dual-beam photoelectron emission microscope.

[0014]FIG. 6 shows a dual-beam photoelectron emission microscope whichincorporates a means for converting the photoelectrons to photons beforethey strike the detector.

[0015]FIG. 7 shows a dual-beam photoelectron emission microscope (701)designed for imaging substrates primarily with photoelectrons.

[0016]FIG. 8 shows a filter for selecting photoelectrons and rejectingreflected electrons.

[0017]FIG. 9 shows a dual-beam photoelectron emission microscopedesigned for imaging substrates primarily with reflected electrons.

[0018]FIG. 10 shows a dual-beam photoelectron emission microscope with ameans for converting the reflected electrons to photons before theyreach the detector.

[0019]FIG. 11 shows a dual-beam photoelectron emission microscope whichincorporates a means for selecting reflected electrons.

[0020]FIG. 12 shows a filter for selecting reflected electrons andrejecting photoelectrons.

[0021]FIG. 13 illustrates a novel method of imaging a substrate with adual-beam photoelectron emission microscope by detecting photoelectrons.

[0022]FIG. 14 illustrates a method of imaging a substrate with adual-beam photoelectron emission microscope which involves filtering theflux of photoelectrons and electrons reflected from the surface of thesubstrate in order to select the photoelectrons.

[0023]FIG. 15 illustrates a novel method of imaging a substrate with adual-beam photoelectron emission microscope by detecting reflectedelectrons.

[0024]FIG. 16 illustrates a method of imaging a substrate with adual-beam photoelectron emission microscope which involves filtering theflux of photoelectrons and electrons reflected from the surface of thesubstrate in order to select the reflected electrons.

[0025]FIG. 17 illustrates a novel method of imaging a substrate with adual-beam photoelectron emission microscope by detecting both reflectedelectrons and photoelectrons.

[0026]FIG. 18 illustrates a novel method of identifying the chemicalcomposition of a defect on a wafer or a reticle.

[0027]FIG. 19 shows a dual-beam secondary electron emission microscopefor imaging substrates primarily with secondary electrons.

[0028]FIG. 20 illustrates a filter which selects secondary electrons andrejects reflected electrons based on their angular distributions.

[0029]FIG. 21 illustrates a novel method of imaging a substrate with adual-beam secondary electron emission microscope.

[0030]FIG. 22 shows a dual-beam secondary electron emission microscopefor imaging substrates primarily with reflected electrons.

[0031]FIG. 23 shows a filter which can select reflected electrons andreject secondary electrons based on their angular distributions.

[0032]FIG. 24 illustrates a novel and useful method of imaging asubstrate with a dual-beam secondary electron emission microscope bydetecting reflected electrons.

[0033]FIG. 25 illustrates a method of imaging a substrate with adual-beam secondary electron emission microscope by detecting bothsecondary electrons and reflected electrons.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] This section describes inventions related to the followingtopics:

[0035] a single-beam photoelectron emission microscope for inspectingwafers and reticles,

[0036] methods of imaging and inspecting wafers or reticles with asingle-beam photoelectron emission microscope,

[0037] a “dual-beam” photoelectron emission microscope which contains anelectron beam as well as a photon beam,

[0038] methods of imaging a substrate with a dual-beam photoelectronemission microscope by detecting photoelectrons,

[0039] methods of imaging a substrate with a dual-beam phototoelectronemission microscope by detecting reflected electrons,

[0040] methods of imaging a substrate with a dual-beam phototoelectronemission microscope by detecting both photoelectrons and reflectedelectrons,

[0041] a dual-electron-beam inspection system with means for filteringto select secondary electrons,

[0042] methods of imaging a substrate with a dual-electron-beaminspection system which involve filtering to select secondary electrons,

[0043] a dual-electron-beam inspection system with means for filteringto select reflected electrons,

[0044] methods of imaging a substrate with a dual-electron-beaminspection system which involve filtering to select reflected electrons,

[0045] methods of imaging a substrate with a dual-electron-beaminspection system which involve filtering to select scattered secondaryand reflected electrons, and

[0046] a method of identifying the chemical composition of a defect on awafer or a reticle.

[0047] Single-Beam Photoelectron Emission Microscope for Wafer orReticle Inspection

[0048] The first invention which we disclose is a single-beamphotoelectron emission microscope for imaging silicon wafers orreticles. A PEEM can offer lower chromatic aberration and thus higherresolution than scanning electron microscopes (which are conventionallyused for wafer and reticle inspection) because photoelectrons have amuch narrower energy spread than secondary electrons. FIG. 1 illustratesa photoelectron emission microscope (101) for wafer or reticleinspection. It includes three main components:

[0049] a means (102) for exposing the wafer or reticle to an influx ofphotons with an energy sufficient to cause photoelectrons to leave thesurface of the wafer or reticle,

[0050] electron optics (103) for focusing the photoelectrons in theplane of a detector, and

[0051] a means (104) for detecting the photoelectrons, thereby imaging aportion of the wafer or reticle.

[0052] The means (102) for exposing the wafer or reticle to an influx ofphotons could be a laser, an arc lamp, or any other light source whichcan emit photons with energy sufficient to cause photoelectrons to leavethe surface of the wafer or the reticle.

[0053] The electron optics 103 used in this and any of the followingembodiments may include various arrangements of lenses (such aselectrostatic lenses, electromagnetic lenses, and combinations thereof).The electron optics shown in the commonly assigned cases previouslyincorporated by reference may also be used.

[0054] The means (104) for detecting the photoelectrons could be adiode, a back-thinned charge-coupled device such as a TDI sensor or anyother electron-sensing device.

[0055]FIG. 2 shows a second embodiment of the photoelectron emissionmicroscope (201) for inspecting wafers or reticles with higher imagequality. It incorporates the three components already shown in FIG. 1:

[0056] a means (202) for exposing the wafer or reticle to an influx ofphotons with an energy sufficient to cause photoelectrons to leave thesurface of the wafer or reticle,

[0057] electron optics (203) for focusing the photoelectrons in theplane of a detector, and

[0058] a means (204) for detecting the photoelectrons, thereby imaging aportion of the wafer or reticle,

[0059] and also a fourth component:

[0060] a means (205) for converting the photoelectrons to photons beforethey strike the detector.

[0061] The means (205) for converting the photoelectrons to photons willimprove image quality. It could consist of a scintillating material, aphosphorescent material, or any other material which will generatephotons when struck by electrons. The detector 204 would then be aphotodetector such as a CCD, TDI sensor, PMT, or any other suitablephotodetecting element. The detector could be closely coupled to orintegral with the scintillator. Means 205 preferably has a large gain.

[0062] Methods of Imaging and Inspecting Wafers or Reticles with aSingle-Beam Photoelectron Emission Microscope

[0063]FIG. 3 illustrates a method of imaging a wafer or reticle with asingle-beam photoelectron emission microscope. It includes the followingsteps:

[0064] exposing the wafer or reticle to an influx of photons withsufficient energy to cause photoelectrons to leave the wafer surface orthe reticle surface (301),

[0065] focusing the photoelectrons to create an image of the wafer orthe reticle in the plane of a detector (302), and

[0066] detecting the photoelectrons (303), thereby imaging a portion ofthe wafer or the reticle.

[0067]FIG. 4 illustrates a method of inspecting a wafer or reticle witha single-beam photoelectron emission microscope. It combines the threesteps described above with reference to FIG. 3:

[0068] exposing the wafer or reticle to an influx of photons withsufficient energy to cause photoelectrons to leave the wafer surface orthe reticle surface (401),

[0069] focusing the photoelectrons to create an image of the wafer orthe reticle in the plane of a detector (402), and

[0070] detecting the photoelectrons, thereby imaging a portion of thewafer or the reticle (403),

[0071] with the following additional step:

[0072] processing the image to detect defects or to classify defects(404).

[0073] In an important variant of the method shown in FIG. 4, at leasttwo materials are visible on the surface of the wafer or the reticle,and the photons which strike the surface have an energy selected toincrease the difference in photoelectron yield between at least two ofthe materials. For example, if the surface of a wafer contains bothpolycrystalline aluminum (which has a work function of about 4.15 eV)and silicon (which has a work function of about 4.8 eV), it would beuseful to select a photon energy of 4.5 eV, in which case the aluminumwould emit photoelectrons and the silicon would not. The image wouldoffer excellent contrast, displaying the aluminum in white and thesilicon in black.

[0074] In another variant of the method shown in FIG. 4, it is possibleto vary the angle at which the influx of photons strikes the substrate.For the sake of design simplicity and for optimization of some imagingmodes, it is useful to direct the influx of photons onto the substrateat a 90 degree angle. However, it is possible to increase sensitivity totopographic defects by directing the influx of photons onto thesubstrate at a smaller angle, even at a grazing angle such as 5 degreesor 10 degrees. Under these conditions, particles and other contaminationshield the area of the substrate behind them from incoming photons. Theshielded areas emit no photoelectrons and therefore appear in theacquired image as elongated shadows which clearly reveal the presence ofdefects. The smaller the angle between the influx of photons and theplane of the substrate, the larger the shadows cast by the defects.

[0075] When directing the influx of photons onto the substrates at anangle of less than 90 degrees, it is possible to optimize image contrastby polarizing the incoming flux of photons. For example, if theapparatus is directing the photons onto the substrate at an angle of,say, 10 degrees, and the influx of photons is horizontally polarized,then the electric field at the surface of the substrate will be verylow. Since the photoemission rate is a function of the electric field atthe surface of the substrate, the photoemission rate will also be verylow. However, for particles or other features that rise above or dipbelow the planar surface of the substrate, the electric field will behigh and therefore the photoemission rate will also be high. Thisstrategy therefore delivers a high degree of image contrast for particledefects, which appear bright against a dark background. If, on the otherhand, the influx of photons is vertically polarized, then the electricfield at the surface of the substrate will be very high, and thephotoemission rate will also be very high, creating a bright image.

[0076] Dual-Beam Photoelectron Emission Microscope for ImagingSubstrates

[0077] One disadvantage of a single-beam photoelectron emissionmicroscope is that as the negatively-charged photoelectrons leave thesubstrate being studied, a positive charge accumulates on the surface ofthe substrate. This residual charge can distort the image. To overcomethis disadvantage, we disclose a dual-beam photoelectron emissionmicroscope which illuminates the substrate with not only a photon beam,but also an electron beam, which prevents the surface of the materialfrom accumulating a strong positive charge. FIG. 5 illustrates thesimplest embodiment of this dual-beam photoelectron emission microscope(501), which includes four main components:

[0078] a means (502) for exposing the substrate to an influx of photonswith an energy sufficient to cause photoelectrons to leave the surfaceof the substrate,

[0079] a means (503) for exposing the substrate to an influx ofelectrons which have both an energy and a current density profileselected to maintain the surface charge present on the substrate at apredetermined level,

[0080] electron optics (504) for focusing the photoelectrons in theplane of a detector, and

[0081] a means (505) for detecting the photoelectrons, thereby imaging aportion of the substrate.

[0082] The means (502) for exposing the wafer or reticle to an influx ofphotons could be a laser, an arc lamp, or any other light source whichcan emit photons with energy sufficient to cause photoelectrons to leavethe surface of the wafer or the reticle.

[0083] The means (503) for exposing the substrate to an influx ofelectrons could be an electron gun.

[0084] The means (505) for detecting the photoelectrons could be acharge-coupled device such as a TDI sensor or any other electron-sensingdevice, as described in more detail previously herein.

[0085]FIG. 6 shows a second embodiment of the dual-beam photoelectronemission microscope (601) for imaging or inspecting substrates withhigher image quality. It incorporates the same four components as theembodiment already shown in FIG. 5:

[0086] a means (602) for exposing the substrate to an influx of photonswith an energy sufficient to cause photoelectrons to leave the surfaceof the substrate,

[0087] a means (603) for exposing the substrate to an influx ofelectrons which have both an energy and a current density profileselected to maintain the surface charge present on the substrate at apredetermined level,

[0088] electron optics (604) for focusing the photoelectrons in theplane of a detector, and

[0089] a means (605) for detecting the photoelectrons,

[0090] with an additional fifth component:

[0091] a means (606) for converting the photoelectrons to photons beforethey strike the detector.

[0092] The means (606) for converting the photoelectrons to photons(thereby improving image quality) could include a scintillatingmaterial, a phosphorescent material, or any other material which willgenerate photons when struck by electrons, as described in more detailpreviously herein.

[0093] In the dual-beam photoelectron emission microscopes shown inFIGS. 5 and 6, the main purpose of the incoming electron beam is toprevent the substrate from gaining a strong positive charge. However,the substrate absorbs only a percentage of the incoming electrons. Theremainder of the incoming electrons are reflected from the surface.These reflected electrons can strike the detector.

[0094] The microscopes shown in FIGS. 5 and 6 may actually generate animage created by a mixture of photoelectrons and reflected electrons. Insome cases, it would be advantageous to create an image primarily withphotoelectrons, which are particularly sensitive to materialdifferences; in other cases, it would be advantageous to create an imageprimarily with reflected electrons, which are particularly sensitive totopography.

[0095] Dual-Beam Photoelectron Emission Microscope for ImagingSubstrates Primarily with Photoelectrons

[0096]FIG. 7 shows a dual-beam photoelectron emission microscope (701)designed for imaging substrates primarily with photoelectrons. Itincorporates the four components shown in FIG. 5:

[0097] a means (702) for exposing the substrate to an influx of photonswith an energy sufficient to cause photoelectrons to leave the surfaceof the substrate,

[0098] a means (703) for exposing the substrate to an influx ofelectrons which have both an energy and a current density profileselected to maintain the surface charge present on the substrate at apredetermined level,

[0099] electron optics (704) for focusing the photoelectrons in theplane of a detector, and

[0100] a means (705) for detecting the photoelectrons, thereby imaging aportion of the substrate,

[0101] with an additional fifth component:

[0102] a means 706 for selecting said most or all of the photoelectrons,or a portion of the photoelectrons, and rejecting most or all of theelectrons reflected from the substrate.

[0103] This fifth component can consist of a filter which selectsphotoelectrons and rejects reflected electrons based on their angulardistribution. FIG. 8 shows one specific example of such a filter 801, ablocking means containing a shaped aperture 802. Both photoelectrons andreflected electrons can leave the surface over a wide range of angles.However, their distribution peaks at different angles. Photoelectronshave the peak of their distribution at an angle normal to the substrate.Reflected electrons have the peak of their distribution at an angle ofreflection which equals the angle of incidence. If we select an angle ofincidence for the electron beam which is far enough from the normal,then the filter can select photoelectrons 803 and reject reflectedelectrons 804 based on their angular distribution.

[0104] Dual-Beam Photoelectron Emission Microscope for ImagingSubstrates Primarily with Reflected Electrons

[0105] The presence of an electron beam, although primarily intended forcharge control, gives us the opportunity to add an imaging mode based onreflected electrons. FIG. 9 shows a dual-beam photoelectron emissionmicroscope (901) designed for imaging substrates with reflectedelectrons. It incorporates the four components shown in FIG. 5:

[0106] a means (902) for exposing the substrate to an influx of photonswith an energy sufficient to cause photoelectrons to leave the surfaceof the substrate,

[0107] a means (903) for exposing the substrate to an influx ofelectrons which have both an energy and a current density profileselected to maintain the surface charge present on the substrate at apredetermined level,

[0108] electron optics (904) for focusing the photoelectrons in theplane of a detector, and

[0109] means (905) for detecting the photoelectrons, thereby imaging aportion of the substrate,

[0110] with an additional fifth component:

[0111] a means (906) for detecting electrons reflected from the surfaceof the substrate.

[0112] The means (906) for detecting reflected electrons could be acharge-coupled device such as a TDI sensor or any other electron-sensingdevice.

[0113] The dual-beam photoelectron emission microscope (901) shown inFIG. 9, like the novel devices described earlier, would achieve betterimage quality if it contained a means for converting the reflectedelectrons to photons before they reach the detector. FIG. 10 shows sucha device, a dual-beam photoelectron emission microscope (1001) whichincorporates the five components also shown in FIG. 9:

[0114] a means (1002) for exposing the substrate to an influx of photonswith an energy sufficient to cause photoelectrons to leave the surfaceof the substrate,

[0115] a means (1003) for exposing the substrate to an influx ofelectrons which have both an energy and a current density profileselected to maintain the surface charge present on the substrate at apredetermined level,

[0116] electron optics (1004) for focusing the photoelectrons in theplane of a detector,

[0117] a means (1005) for detecting the photoelectrons, thereby imaginga portion of the substrate, and

[0118] a means (1006) for detecting electrons reflected from the surfaceof the substrate,

[0119] with an additional sixth component:

[0120] a means (1007) for converting the reflected electrons to photonsbefore they strike the detector.

[0121] The means (1007) for converting the photoelectrons to photons(thereby improving image quality) could consist of a scintillatingmaterial, a phosphorescent material, or any other material which willgenerate photons when struck by electrons.

[0122] In the dual-beam photoelectron emission microscopes shown inFIGS. 9 and 10, the means (906 and 1006) for detecting reflectedelectrons detects photoelectrons as well as reflected electrons. FIG. 11shows an embodiment of the invention which creates images basedpreferentially on reflected electrons. It contains the five componentsshown in FIG. 9:

[0123] a means (1102) for exposing the substrate to an influx of photonswith an energy sufficient to cause photoelectrons to leave the surfaceof the substrate,

[0124] a means (1103) for exposing the substrate to an influx ofelectrons which have both an energy and a current density profileselected to maintain the surface charge present on the substrate at apredetermined level,

[0125] electron optics (1104) for focusing the photoelectrons in theplane of a detector,

[0126] a means (1105) for detecting the photoelectrons, thereby imaginga portion of the substrate, and

[0127] a means (1106) for detecting electrons reflected from the surfaceof the substrate,

[0128] with an additional sixth component:

[0129] a means (1107) for selecting said most or all of the reflectedelectrons, or a portion of the reflected electrons, and rejecting mostor all of the photoelectrons emitted from the substrate.

[0130] This sixth component can consist of a filter which selectsreflected electrons and rejects photoelectrons based on their angulardistribution. FIG. 12 shows one specific example of such a filter 1201,a blocking means containing a shaped aperture 1202. As explained abovewith reference to FIG. 8, the filter can select reflected electrons 1203and reject photoelectrons 1204 based on their angular distributionbecause the reflected electrons have the peak of their distribution atan angle of reflection which equals their angle of incidence, and thephotoelectrons have the peak of their distribution normal to thesubstrate.

[0131] Having completed our disclosure of innovative hardware fordual-beam photoelectron emission microscopes, we now describe novelmethods of applying those systems, first by detecting primarilyphotoelectrons, then by detecting primarily reflected electrons, andfinally by detecting both photoelectrons and reflected electrons.

[0132] Methods of Imaging a Substrate with a Dual-Beam PhotoelectronEmission Microscope by Detecting Photoelectrons

[0133]FIG. 13 illustrates a novel method of imaging a substrate with adual-beam photoelectron emission microscope by detecting photoelectrons.It includes the following steps:

[0134] exposing the substrate to an influx of photons with sufficientenergy to cause photoelectrons to leave the surface of the substrate(1301),

[0135] exposing the substrate to an influx of electrons which have bothan energy and a current density profile selected to maintain the surfacecharge present on the substrate at a predetermined level (1302),

[0136] focusing the photoelectrons to create an image of the substratein the plane of a detector (1303), and

[0137] detecting the photoelectrons (1304), thereby imaging a portion ofthe substrate.

[0138] The novelty of this step lies in step (1302), where an electronbeam prevents the accumulation of a strong positive charge on thesubstrate.

[0139] In an important variant of this method, at least two materialsare visible on the surface of the substrate, and the photons whichstrike the surface have an energy selected to increase the difference inphotoelectron yield between at least two of the materials. An earliersection gave the example of choosing a photon energy between the workfunction of aluminum and the work function of silicon in order toincrease image contrast; that example applies here as well.

[0140] In other variants of the method shown in FIG. 13, it is possibleto expose the substrate to the influx of photons and the influx ofelectrons either concurrently or alternately.

[0141] In yet another variant of the method shown in FIG. 13, it ispossible to limit the accumulation of positive charge on the wafer mosteffectively by exposing the substrate to electrons over a relativelylarge area and to photons over a relatively small area confined withinthe larger area exposed to electrons.

[0142] The method shown in FIG. 13 may detect some reflected electrons(i.e., electrons in the charge control beam reflected from the surfaceof the substrate) as well as photoelectrons. To produce an image withoptimal contrast between different materials, it will be useful tomaximize the percentage of photoelectrons and minimize the percentage ofreflected electrons which strike the detector. To this end, we disclosethe method shown in FIG. 14:

[0143] exposing the substrate to an influx of photons with sufficientenergy to cause photoelectrons to leave the surface of the substrate(1401),

[0144] exposing the substrate to an influx of electrons which have bothan energy and a current density profile selected to maintain the surfacecharge present on the substrate at a predetermined level (1402),

[0145] filtering the flux of photoelectrons and electrons reflected fromthe surface of the substrate in order to select the photoelectrons, or aportion of the photoelectrons, and to reject most or all reflectedelectrons (1403),

[0146] focusing the photoelectrons to create an image of the substratein the plane of a detector (1404), and

[0147] detecting the photoelectrons (1405), thereby imaging a portion ofthe substrate,

[0148] One preferred method of achieving the filtering step (1403) is tofilter the flux of photoelectrons and reflected electrons based on theirangular distribution from the surface, as discussed above with referenceto FIG. 8.

[0149] In yet another variant of the method shown in FIG. 13, thesurface of the substrate is made up of at least two materials, and thephotons which strike the surface have an energy selected to increase thedifference in photoelectron yield between at least two of the materials,as described earlier. In yet another variant of the method shown in FIG.13, it is possible to direct the influx of photons on the substrate at a90 degree angle for some imaging modes or at some smaller angle forother imaging modes, as described earlier. Likewise, it can be useful topolarize the influx of photons either vertically or horizontally. Anexplanation of why these variants are useful appears above in thesection on Methods of Imaging and Inspecting Wafers or Reticles with aSingle-Beam Photoelectron Emission Microscope.

[0150] Methods of Imaging a Substrate with a Dual-Beam PhotoelectronEmission Microscope by Detecting Reflected Electrons

[0151]FIG. 15 illustrates a novel method of imaging a substrate with adual-beam photoelectron emission microscope by detecting reflectedelectrons. It includes the following steps:

[0152] exposing the substrate to an influx of photons with sufficientenergy to cause photoelectrons to leave the surface of the substrate(1501),

[0153] exposing the substrate to an influx of electrons which have bothan energy and a current density profile selected to maintain the surfacecharge present on the substrate at a predetermined level (1502),

[0154] focusing electrons which are reflected from the substrate in theplane of a detector (1503), and

[0155] detecting the reflected electrons (1504), thereby imaging aportion of the substrate.

[0156] The novelty of this method lies in steps (1503) and (1504), usinga dual-beam photoemission electron microscope to create an image fromelectrons in the charge control beam which are reflected by thesubstrate. This image can be extremely sensitive to topography.

[0157] In variants of the method shown in FIG. 15, it is possible toexpose the substrate to the influx of photons and the influx ofelectrons either concurrently or alternately.

[0158] In yet another variant of the method shown in FIG. 15, it ispossible to limit the accumulation of positive charge on the wafereffectively by exposing the substrate to electrons over a relativelylarge area and to photons over a relatively small area confined withinthe larger area exposed to electrons.

[0159] The method shown in FIG. 15 may detect some photoelectrons aswell as reflected electrons. To produce an image with optimalsensitivity to topography, it will be useful to maximize the percentageof reflected electrons and minimize the percentage of photoelectronswhich strike the detector. To this end, we disclose the method shown inFIG. 16:

[0160] exposing the substrate to an influx of photons with sufficientenergy to cause photoelectrons to leave the surface of the substrate(1601),

[0161] exposing the substrate to an influx of electrons which have bothan energy and a current density profile selected to maintain the surfacecharge present on the substrate at a predetermined level (1602),

[0162] filtering the flux of photoelectrons and electrons reflected fromthe surface of the substrate in order to select the reflected electrons,or a portion of the reflected electrons, and to reject most or all ofthe photoelectrons (1603),

[0163] focusing reflected electrons in the plane of a detector (1604),and

[0164] detecting the reflected electrons (1605), thereby imaging aportion of the substrate.

[0165] One preferred method of carrying out the filtering in step (1603)is to select the reflected electrons, or a portion of the reflectedelectrons, based on their angular distribution from the surface of thesubstrate. As explained above, the reflected electrons have their peakof distribution at a specular angle (i.e., at an angle of reflectionwhich equals their angle of incidence), whereas the photoelectrons havetheir peak of distribution normal to the substrate.

[0166] In a variation of the method described in the previous paragraph,the filtering rejects most or all of the reflected electrons which arereflected at or near the specular angle and selects most or allreflected electrons which are scattered away from the specular angle.This method gives high sensitivity to particles or other contaminationdefects on the surface which scatter incoming electrons.

[0167] Methods of Imaging a Substrate with a Dual-Beam PhotoelectronEmission Microscope by Detecting Both Photoelectrons and ReflectedElectrons

[0168]FIG. 17 illustrates a novel method of imaging a substrate with adual-beam photoelectron emission microscope by detecting bothphotoelectrons and reflected electrons. It includes the following steps:

[0169] exposing the substrate to an influx of photons with energyselected to cause photoelectrons to leave the substrate (1701),

[0170] exposing the substrate to an influx of electrons with both anenergy and a current density profile selected to maintain the surfacecharge present on the substrate at a predetermined level (1702),

[0171] focusing electrons reflected from the surface of the substrate inthe plane of a detector (1703),

[0172] focusing photoelectrons in the plane of a detector (1704), and

[0173] detecting the photoelectrons and reflected electrons (1705),thereby imaging a portion of the substrate.

[0174] In a variation of the method shown in FIG. 17, it can be usefulto position the filter so that it increases sensitivity to defects anddecreases sensitivity to non-defective parts of the surface. Areas onthe substrate which are free of particle contamination tend to reflectelectrons at or near the specular angle and to emit photoelectrons at anangle perpendicular to the substrate. To decrease sensitivity to thoseareas, it is useful to position a filter so that it rejects most or allof the reflected electrons which are reflected at or near the specularangle and most or all of the photoelectrons which are emittedperpendicular to the surface of the substrate. On the other hand,particles and other contamination defects tend to scatter reflectedelectrons away from the specular angle and to emit photoelectrons atangles other than perpendicular to the substrate. To increasesensitivity to those defects, it is useful to position the filter sothat it selects most or all of the reflected electrons which arescattered away from the specular angle and most or all of thephotoelectrons which are emitted at angles other than perpendicular tothe surface. Under these circumstances, the image will offer excellentcontrast for contamination defects, which will appear white against adark background.

[0175] Methods of Identifying the Chemical Composition of a Defect witha Photoelectron Emission Microscope

[0176] A photoelectron emission microscope for inspecting substratesgives a new capability: a method of identifying the chemical compositionof a defect. In a wafer fab or a mask shop, this capability can helpengineers to identify the source of a defect so they can quickly correcta yield-limiting problem. FIG. 18 illustrates this method:

[0177] exposing the defect to an influx of photons with energy below theenergy required to cause photoelectrons to leave the defect (1801),

[0178] increasing the energy of the photons in discrete steps (1802),

[0179] monitoring the photoelectron yield from the defect after eachstep (1803), and

[0180] identifying the chemical composition of the defect on the basisof the photon energy at which the photoelectron yield increasessubstantially (1804).

[0181] The photoelectron yield will increase substantially when theenergy of the photons reaches the work function of the material fromwhich the defect is made. The value of that energy provides a clue tothe chemical makeup of the defect because the work functions ofmaterials used in semiconductor manufacturing are widely known.

[0182] The novel apparati and methods we have described for filteringphotoelectrons and reflected electrons in a photoemission electronmicroscope are also novel and useful when applied to inspection ofsubstrates with a dual-beam electron microscope. We will now disclosethose inventions.

[0183] Dual-Beam Secondary Electron Emission Microscope for ImagingSubstrates Primarily with Secondary Electrons

[0184]FIG. 19 shows a dual-beam secondary electron emission microscope(1901) for imaging substrates primarily with secondary electrons. Itincludes the following components:

[0185] means (1902) for exposing the substrate to an influx ofrelatively high-energy electrons, with energy selected to causesecondary electrons to leave the substrate,

[0186] means (1903) for exposing the substrate to an influx ofrelatively low-energy electrons, with both an energy and a currentdensity profile selected to maintain surface charge present on thesubstrate at a predetermined level,

[0187] means (1904) for selecting most or all of the secondaryelectrons, or a portion of the secondary electrons, and rejecting mostor all of the relatively low-energy electrons reflected from thesubstrate, and

[0188] means (1905) for detecting the secondary electrons, therebyimaging a portion of said substrate.

[0189] The novelty of the invention shown in FIG. 19 lies in (1904), themeans for selecting secondary electrons and rejecting reflectedelectrons. This means (1904) can consist of a filter which selects mostor all of the secondary electrons and rejects most or all of thereflected electrons based on their angular distributions. FIG. 20 showsone possible embodiment of this filter, a blocking means 2001 containingan aperture 2002.

[0190] Methods of Imaging a Substrate with a Dual-Beam SecondaryElectron Emission Microscope by Detecting Photoelectrons

[0191]FIG. 21 illustrates a novel and useful method of imaging asubstrate with a dual-beam secondary electron emission microscope. Itincludes the following five steps:

[0192] exposing the substrate to an influx of relatively high-energyelectrons, with energy selected to cause secondary electrons to leavethe substrate (2101),

[0193] exposing the substrate to an influx of relatively low-energyelectrons, with both an energy and a current density profile selected tomaintain surface charge present on the substrate at a predeterminedlevel (2102),

[0194] filtering the flux of secondary electrons and low-energyelectrons reflected from the surface of the substrate in order to selectmost or all of the secondary electrons, or a portion of the secondaryelectrons, and to reject most or all of the reflected electrons (2103),

[0195] focusing the secondary electrons to create an image of thesubstrate in the plane of a detector (2104), and

[0196] detecting the secondary electrons, thereby imaging a portion ofthe substrate (2105).

[0197] The novelty of this method lies in step (2103), filtering theflux of secondary electrons and reflected electrons to select secondaryelectrons and reject reflected electrons. One preferred method offiltering the flux (2103) is to select the secondary electrons, or aportion of the secondary electrons, based on their angular distributionfrom the surface of the substrate. This method is possible is becausethe secondary electrons (like the photoelectrons in a photoelectronemission microscope) have the peak of their distribution normal to thesurface of the substrate. The reflected electrons have the peak of theirdistribution at an angle of reflection which equals their angle ofincidence.

[0198] Dual-Beam Secondary Electron Emission Microscope for ImagingSubstrates Primarily with Reflected Electrons

[0199]FIG. 22 shows a dual-beam secondary electron emission microscope(2201) for imaging substrates primarily with reflected electrons. Itincludes the following components:

[0200] means (2202) for exposing the substrate to an influx ofrelatively high-energy electrons, with energy selected to causesecondary electrons to leave the substrate,

[0201] means (2203) for exposing said substrate to an influx ofrelatively low-energy electrons, with both an energy and a currentdensity profile selected to maintain surface charge present on thesubstrate at a predetermined level,

[0202] means (2204) for selecting most or all of the relativelylow-energy electrons reflected from the substrate, or a portion of thosereflected electrons, and rejecting most or all of the secondaryelectrons, and

[0203] means (2205) for detecting the reflected electrons, therebyimaging a portion of said substrate.

[0204] The novelty of the invention shown in FIG. 22 lies in (2204), themeans for selecting reflected electrons and rejecting secondaryelectrons. This means (2204) can consist of a filter which selects mostor all of the reflected electrons and rejects most or all of thesecondary electrons based on their angular distributions. FIG. 23 showsone possible embodiment of this filter, a blocking means 2301 containingan aperture 2302.

[0205] Methods of Imaging a Substrate with a Dual-Beam SecondaryElectron Emission Microscope by Detecting Reflected Electrons

[0206]FIG. 24 illustrates a novel and useful method of imaging asubstrate with a dual-beam secondary electron emission microscope bydetecting reflected electrons. It includes the following five steps:

[0207] exposing the substrate to an influx of relatively high-energyelectrons, with energy selected to cause secondary electrons to leavethe substrate (2401),

[0208] exposing the substrate to an influx of relatively low-energyelectrons, with both an energy and a current density profile selected tomaintain surface charge present on the substrate at a predeterminedlevel (2402),

[0209] filtering the flux of secondary electrons and low-energyelectrons reflected from the surface of the substrate in order to selectmost or all of the reflected electrons, or a portion of the reflectedelectrons, and to reject most or all of the secondary electrons (2403),

[0210] focusing the reflected electrons to create an image of thesubstrate in the plane of a detector (2404), and

[0211] detecting the reflected electrons, thereby imaging a portion ofthe substrate (2405).

[0212] The novelty of this method lies in step (2403), filtering theflux of reflected electrons and secondary electrons to select reflectedelectrons and reject secondary electrons. One preferred method offiltering the flux (2403) is to select the reflected electrons, or aportion of the reflected electrons, based on their angular distributionfrom the surface of the substrate. This method is possible because thereflected electrons have the peak of their distribution at an angle ofreflection which equals the angle of incidence. The secondary electrons(like the photoelectrons in a photoelectron emission microscope) havethe peak of their distribution normal to the surface of the substrate.

[0213] A different way to filter the reflected electrons based on theirangular distribution is to reject most or all of the reflected electronswhich are reflected at or near the specular angle and to select most orall of the reflected electrons which are scattered away from thespecular angle. This method delivers superior sensitivity to particlesor other contamination defects which scatter electrons.

[0214] Method of Imaging a Substrate with a Dual-Beam Secondary ElectronEmission Microscope by Detecting both Secondary Electrons and ReflectedElectrons

[0215]FIG. 25 illustrates a method of imaging a substrate with asecondary electron emission microscope by detecting both secondary andreflected electrons. The distinguishing feature of this method is toposition the filter so that it increases sensitivity to defects anddecreases sensitivity to non-defective parts of the surface. Areas onthe substrate which are free of particle contamination tend to reflectincoming low-energy electrons at or near the specular angle and to emitsecondary electrons at an angle perpendicular to the substrate.Therefore, it is possible to decrease sensitivity to those areas bypositioning a filter so that it rejects electrons which are reflected ator near the specular angle and secondary electrons which are emittedperpendicular to the surface of the substrate. However, particles andother contamination defects tend to scatter reflected electrons awayfrom the specular angle and to emit photoelectrons at angles other thanperpendicular to the substrate. One can increase sensitivity to thesedefects by positioning the filter so that it selects reflected electronswhich are scattered away from the specular angle and photoelectronswhich are emitted at angles other than perpendicular to the surface. Thecontamination defects will then appear white against a dark backgroundin the acquired image.

[0216] In total, the method includes the following steps:

[0217] exposing the substrate to an influx of relatively high-energyelectrons with an energy selected to cause secondary electrons to leavethe substrate (2501),

[0218] exposing the substrate to an influx of relatively low-energyelectrons with both an energy and a current density profile selected tomaintain surface charge present on the substrate at a predeterminedlevel (2502),

[0219] filtering the secondary electrons and the portion of relativelylow-energy electrons which are reflected from the surface of thesubstrate, in order to select most or all of the secondary electronswhich are emitted at angles other than perpendicular to the substrateand most or all of the reflected electrons which are scattered away fromthe specular angle, and to reject most or all of the secondary electronswhich are emitted at an angle perpendicular to the substrate and most orall of the reflected electrons which are scattered at the specular angle(2503),

[0220] focusing the selected secondary and reflected electrons to createan image of the substrate in the plane of a detector (2504),

[0221] detecting the selected secondary and reflected electrons, therebyimaging a portion of the substrate (2505).

[0222] The invention described herein is intended for the inspection ofsemiconductor wafers, photomasks, or other patterned or unpatternedsubstrates. More broadly, it can be applied to the imaging or inspectionof any kind of substrate with a photoemission electron microscope or asecondary electron emission microscope. Although the invention has beendescribed in relation to various implementations, together withmodifications, variations, and extensions thereof, otherimplementations, modifications, variations and extensions are within thescope of the invention. Other embodiments may be apparent to thoseskilled in the art from consideration of the specification and inventiondisclosed herein. The invention is therefore not limited by thedescription contained herein or by the drawings, but only by the claimsand their equivalents.

What is claimed is:
 1. An apparatus for inspecting a wafer or a reticle,comprising: a) means for exposing said wafer or said reticle to aninflux of photons, said photons having an energy selected to causephotoelectrons to leave said wafer or said reticle, b) electron opticsfor focusing said photoelectrons in the plane of a detection means, andc) means for detecting said photoelectrons, thereby imaging a portion ofsaid wafer or said reticle.
 2. The apparatus of claim 1, wherein saidmeans for exposing said substrate to an influx of photons is a laser. 3.The apparatus of claim 1, wherein said means for exposing substrate toan influx of photons is an arc lamp.
 4. The apparatus of claim 1,wherein said means for detecting said photoelectrons is a charge-coupleddevice.
 5. The apparatus of claim 1, wherein said means for detectingsaid photoelectrons is a TDI sensor.
 6. The apparatus of claim 1,further comprising a means for converting said photoelectrons tophotons.
 7. The apparatus of claim 1, wherein said means for convertingsaid photoelectrons to photons is a scintillating material.
 8. Theapparatus of claim 1, wherein said means for converting saidphotoelectrons to photons is a phosphorescent material.
 9. A method ofimaging a wafer or a reticle to find defects, comprising: a) exposingsaid wafer or said reticle to an influx of photons, said photons havingan energy selected to cause photoelectrons to leave the surface of saidwafer or said reticle, b) focusing said photoelectrons to create animage of said wafer or said reticle in the plane of a detector, and c)detecting said photoelectrons, thereby imaging a portion of said waferor said reticle.
 10. The method of claim 9, further comprising: d)processing the image to detect defects or to classify defects.
 11. Themethod of claim 10, wherein a) said wafer or reticle comprises at leasttwo materials, and b) said photons have an energy selected to increasethe difference in photoelectron yield between at least two of saidmaterials.
 12. The method of claim 10, wherein said influx of photons isoriented at a 90 degree angle to the substrate.
 13. The method of claim10, wherein said influx of photons is oriented at an angle of less than90 degrees to the substrate.
 14. The method of claim 10, wherein saidinflux of photons is vertically polarized.
 15. The method of claim 10wherein said influx of photons is horizontally polarized.
 16. Anapparatus for imaging a substrate, comprising: a) means for exposingsaid substrate to an influx of photons, said photons having an energyselected to cause photoelectrons to leave said substrate, b) means forexposing said substrate to an influx of electrons, said electrons havingboth an energy and a current density profile selected to maintainsurface charge present on said substrate at a predetermined level, c)imaging electron optics for focusing said photoelectrons in the plane ofa detector, and d) means for detecting said photoelectrons, therebyimaging a portion of said substrate.
 17. The apparatus of claim 16,wherein said means for exposing said substrate to an influx of photonsis a laser.
 18. The apparatus of claim 16, wherein said means forexposing substrate to an influx of photons is an arc lamp.
 19. Theapparatus of claim 16, wherein said means for detecting saidphotoelectrons is a charge-coupled device.
 20. The apparatus of claim16, wherein said means for detecting said photoelectrons is a TDIsensor.
 21. The apparatus of claim 16, further comprising a means forconverting said photoelectrons to photons.
 22. The apparatus of claim21, wherein said means for converting said photoelectrons to photons isa scintillator.
 23. The apparatus of claim 21, wherein said means forconverting said photoelectrons to photons is a phosphorescent material.24. The apparatus of claim 16, further comprising: a) means forselecting most or all of said photoelectrons, or a portion of saidphotoelectrons, and rejecting most or all of said electrons reflectedfrom said substrate.
 25. The apparatus of claim 24, wherein said meansfor selecting most or all of said photoelectrons, or a portion of saidphotoelectrons, and rejecting most or all of said reflected electrons isa filter which selects said photoelectrons and rejects said reflectedelectrons based on the angular distribution of said photoelectrons andreflected electrons.
 26. The apparatus of claim 25, wherein said filterincludes a blocking means containing a shaped aperture.
 27. Theapparatus of claim 16, further comprising: a) means for detectingelectrons reflected from the surface of said substrate, thereby imaginga portion of said substrate.
 28. The apparatus of claim 27, wherein saidmeans for detecting said reflected electrons is a charge-coupled device.29. The apparatus of claim 27, wherein said means for detecting saidreflected electrons is a TDI sensor.
 30. The apparatus of claim 27,further comprising a means for converting said reflected electrons tophotons.
 31. The apparatus of claim 30, wherein said means forconverting said reflected electrons to photons is a scintillator. 32.The apparatus of claim 30, wherein said means for converting saidreflected electrons to photons is a phosphorescent material.
 33. Theapparatus of claim 27, further comprising: a) means for selecting saidmost or all of electrons reflected from the substrate, or a portion ofsaid reflected electrons, and rejecting most or all of saidphotoelectrons.
 34. The apparatus of claim 33, wherein said means forselecting most or all of said reflected electrons, or a portion of saidreflected electrons, and rejecting most or all of said photoelectrons isa filter which selects said reflected electrons and rejects saidphotoelectrons based on the angular distribution of said reflectedelectrons and photoelectrons.
 35. The apparatus of claim 34, whereinsaid filter includes a blocking means containing a shaped aperture. 36.A method of imaging a substrate, comprising: a) exposing said substrateto an influx of photons, said photons having an energy selected to causephotoelectrons to leave said substrate, b) exposing said substrate to aninflux of electrons, said electrons having both an energy and a currentdensity profile selected to maintain surface charge present on saidsubstrate at a predetermined level, c) focusing said photoelectrons tocreate an image of said substrate in the plane of a detector, and d)detecting said photoelectrons, thereby imaging a portion of saidsubstrate.
 37. The method of claim 36, wherein a) said substratecomprises at least two materials, and b) said photons have an energyselected to increase the difference in photoelectron yield between atleast two of said materials.
 38. The method of claim 36, wherein saidsubstrate is concurrently exposed to said influx of photons and saidinflux of electrons.
 39. The method of claim 36, wherein said substrateis alternately exposed to said influx of photons and said influx ofelectrons.
 40. The method of claim 36, wherein said substrate is exposedto said influx of photons over a first area, said substrate is exposedto said influx of electrons over a second area, and said first area issubstantially contained within said second area.
 41. The method of claim36, further comprising the additional step interposed between step b)and step c): a) filtering the flux of photoelectrons and electronsreflected from the surface of said substrate in order to select saidphotoelectrons, or a portion of said photoelectrons, and to reject mostor all of said reflected electrons.
 42. The method of claim 37, whereinsaid filtering is achieved by selecting said photoelectrons based ontheir angular distribution from said surface of said substrate.
 43. Themethod of claim 36, wherein a) said surface of said substrate comprisesat least two materials, and b) said photons have an energy selected toincrease the difference in photoelectron yield between at least two ofsaid materials.
 44. The method of claim 36, wherein said influx ofphotons is oriented at a 90 degree angle to the substrate.
 45. Themethod of claim 36, wherein said influx of photons is oriented at anangle of less than 90 degrees to the substrate.
 46. The method of claim36, wherein said influx of photons is vertically polarized.
 47. Themethod of claim 36 wherein said influx of photons is horizontallypolarized.
 48. A method of imaging a substrate, comprising: a) exposingsaid substrate to an influx of photons, said photons having an energyselected to cause photoelectrons to leave said substrate, b) exposingsaid substrate to an influx of electrons, said electrons having both anenergy and a current density profile selected to maintain surface chargepresent on said substrate at a predetermined level, c) focusing theportion of said influx of electrons which are reflected from saidsubstrate to create an image of said substrate in the plane of adetector, and d) detecting the portion of said influx of electrons whichare reflected from said substrate, thereby imaging a portion of saidsubstrate.
 49. The method of claim 48, wherein said substrate isconcurrently exposed to said influx of photons and said influx ofelectrons.
 50. The method of claim 48, wherein said substrate isalternately exposed to said influx of photons and said influx ofelectrons.
 51. The method of claim 48, wherein said substrate is exposedto said influx of photons over a first area, said substrate is exposedto said influx of electrons over a second area, and said first area issubstantially contained within said second area.
 52. The method of claim48, further comprising the following step, interposed between step b)and step c): a) filtering the flux of photoelectrons and electronsreflected from the surface of said substrate in order to select saidreflected electrons, or a portion of said reflected electrons, and toreject most or all of said photoelectrons.
 53. The method of claim 52,wherein said filtering is achieved by selecting said reflectedelectrons, or a portion of said reflected electrons, based on theirangular distribution from the surface of said substrate.
 54. The methodof claim 53 wherein said filtering rejects most or all reflectedelectrons which are reflected at or near the specular angle and selectsmost or all reflected electrons which are scattered away from thespecular angle.
 55. A method of imaging a substrate, comprising: a)exposing said substrate to an influx of photons, said photons having anenergy selected to cause photoelectrons to leave said substrate, b)exposing said substrate to an influx of electrons, said electrons havingboth an energy and a current density profile selected to maintainsurface charge present on said substrate at a predetermined level, c)focusing the portion of said influx of electrons which are reflectedfrom the surface of said substrate in the plane of a detector, d)focusing said photoelectrons in the plane of a detector, and e)detecting said photoelectrons and reflected electrons, thereby imaging aportion of said substrate.
 56. The method of claim 55, furthercomprising: a) filtering said reflected electrons and saidphotoelectrons to reject most or all of said reflected electrons whichare reflected at or near the specular angle and most or all of saidphotoelectrons which are emitted perpendicular to the surface of thesubstrate, and to select most or all of said reflected electrons whichare scattered away from the specular angle and/or most or all of saidphotoelectrons which are emitted at angles other than perpendicular tothe surface of the substrate.
 57. A method of identifying the chemicalcomposition of a defect on a wafer or a reticle, comprising: a) exposingsaid defect to an influx of photons, said photons having an energy belowthe energy required to cause photoelectrons to leave said defect, b)increasing the energy of said photons in discrete steps, c) monitoringthe photoelectron yield from said defect after each step, and d)identifying the chemical composition of said defect on the basis of thephoton energy at which said photoelectron yield increases substantially.58. An apparatus for imaging a substrate, comprising: a) means forexposing said substrate to an influx of relatively high-energyelectrons, said high-energy electrons having an energy selected to causesecondary electrons to leave said substrate, b) means for exposing saidsubstrate to an influx of relatively low-energy electrons, saidlow-energy electrons having both an energy and a current density profileselected to maintain surface charge present on said substrate at apredetermined level, c) means for selecting most or all of saidsecondary electrons, or a portion of said secondary electrons, andrejecting most or all of said relatively low-energy electrons reflectedfrom said substrate, and d) means for detecting said secondaryelectrons, thereby imaging a portion of said substrate.
 59. Theapparatus of claim 58, wherein said means for selecting said secondaryelectrons and rejecting said reflected low-energy electrons includes afilter which selects most or all of said secondary electrons and rejectsmost or all of said reflected low-energy electrons based on the angulardistribution of said secondary electrons and said reflected low-energyelectrons.
 60. The apparatus of claim 59, wherein said filter includes ablocking means containing an aperture.
 61. A method of imaging asubstrate, comprising: a) exposing said substrate to an influx ofrelatively high-energy electrons, said high-energy electrons having anenergy selected to cause secondary electrons to leave said substrate, b)exposing said substrate to an influx of relatively low-energy electrons,said electrons having both an energy and a current density profileselected to maintain surface charge present on said substrate at apredetermined level, c) filtering the flux of said secondary electronsand said low-energy electrons reflected from the surface of saidsubstrate in order to select most or all of said secondary electrons, ora portion of said secondary electrons, and to reject most or all of saidreflected electrons, d) focusing said secondary electrons to create animage of said substrate in the plane of a detector, and e) detectingsaid secondary electrons, thereby imaging a portion of said substrate.62. The method of claim 61, wherein said filtering is achieved byselecting said secondary electrons, or a portion of said secondaryelectrons, based on their angular distribution from the surface of saidsubstrate.
 63. An apparatus for imaging a substrate, comprising: a)means for exposing said substrate to an influx of relatively high-energyelectrons, said high-energy electrons having an energy selected to causesecondary electrons to leave said substrate, b) means for exposing saidsubstrate to an influx of relatively low-energy electrons, saidlow-energy electrons having both an energy and a current density profileselected to maintain surface charge present on said substrate at apredetermined level, c) means for selecting most or all of saidlow-energy electrons reflected from said substrate, or a portion of saidlow-energy electrons reflected from said substrate, and rejecting mostor all of said secondary electrons, and d) means for detecting saidreflected low-energy electrons, thereby imaging a portion of saidsubstrate.
 64. The apparatus of claim 63, wherein said means forselecting said reflected low-energy electrons and rejecting saidsecondary electrons is a filter which selects said reflected low-energyelectrons and rejects said secondary electrons based on the angulardistribution of said reflected low-energy electrons and said secondaryelectrons.
 65. The apparatus of claim 64, wherein said filter includes ablocking means containing an aperture.
 66. A method of imaging asubstrate, comprising: a) exposing said substrate to an influx ofrelatively high-energy electrons, said high-energy electrons having anenergy selected to cause secondary electrons to leave said substrate, b)exposing said substrate to an influx of relatively low-energy electrons,said electrons having both an energy and a current density profileselected to maintain surface charge present on said substrate at apredetermined level, c) filtering the flux of said secondary electronsand said low-energy electrons reflected from the surface of saidsubstrate in order to select most or all of said reflected low-energyelectrons, or a portion of said reflected low-energy electrons, and toreject most or all of said secondary electrons, d) focusing saidreflected low-energy electrons to create an image of said substrate inthe plane of a detector, and e) detecting said reflected low-energyelectrons, thereby imaging a portion of said substrate.
 67. The methodof claim 66, wherein said filtering is achieved by selecting saidreflected low-energy electrons, or a portion of said reflectedlow-energy electrons, based on their angular distribution from thesurface of said substrate.
 68. The method of claim 58 wherein saidfiltering rejects most or all of said reflected low-energy electronswhich are reflected at or near the specular angle and selects most orall of said reflected low-energy electrons which are scattered away fromthe specular angle.
 69. A method of imaging a substrate, comprising: a)exposing said substrate to an influx of relatively high-energyelectrons, said high-energy electrons having an energy selected to causesecondary electrons to leave said substrate, b) exposing said substrateto an influx of relatively low-energy electrons, said electrons havingboth an energy and a current density profile selected to maintainsurface charge present on said substrate at a predetermined level, c)filtering said secondary electrons and the portion of said relativelylow-energy electrons which are reflected from the surface of saidsubstrate, in order to select most or all of said secondary electronswhich are emitted at angles other than perpendicular to the substrateand most or all of said reflected electrons which are scattered awayfrom the specular angle, and to reject most or all of said secondaryelectrons which are emitted at an angle perpendicular to the substrateand most or all of said reflected electrons which are scattered at thespecular angle, d) focusing said selected secondary electrons and saidselected reflected electrons to create an image of said substrate in theplane of a detector, e) detecting said selected secondary electrons andsaid selected reflected electrons, thereby imaging a portion of saidsubstrate.